Foams composed of polypropylene and other propylene resins are generally excellent in properties such as heat resistance, strength and rigidity as compared with those of polyethylene foam, so that the use thereof is expanding in the fields of high-temperature heat insulators, packaging materials, building materials and lightweight structural materials.
The conventional polypropylene foam has an expansion ratio as high as 15 to 40 and is a soft foam composed mainly of a soft propylene random copolymer. However, the polypropylene foam is required to be lightweight and have high rigidity for use in automotive interior finish base materials (aggregate).
A variety of base materials are known for use as automotive interior finish base materials, for example, automotive roof base materials, which include a corrugated paper board, a composite material of polyurethane foam containing a glass fiber, a resin felt composed of a fiber sheet and a thermosetting resin bound together, a material composed mainly of a glass fiber with the use of polyethylene or polypropylene as a binder and Dylark (trade name) comprised of polystyrene foam.
However, all the above base materials have respective merits and demerits and none of them satisfies all the requirements on the properties of the automotive molded roof material which include that it must be lightweight, that its rigidity must be high, that it must have excellent dimensional stability, that its acoustic characteristic must be excellent, that its cost must be low, that it must have freedom in molding, that it must be free from environmental pollution when molding, that it can be recycled (recyclability), that its design variation must be wide (designability) and that it can be easily mounted. In particular, greater importance would be placed in the future on the recyclability from the viewpoint of the global environment problem. At the present time, although some base materials for use as automotive roof materials are known which satisfy the recyclability requirement, they have not acquired the mainstream status because of their other problems.
Japanese Patent Laid-open Publication No. 5(1993)-70621 proposed the following process for producing a foamed polypropylene molding product. That is, the process for producing a foamed polypropylene molding product comprises sheeting a foamable sheet forming composition comprising a propylene resin composed of a propylene block copolymer, a radical initiator, a crosslinking auxiliary and a foaming agent at a temperature at which the foaming agent does not decompose into a foamable sheet, lining one side of the resultant foamable sheet with a backing sheet or laminating a cover sheet having the function of a backing sheet thereto so as to integrate these and irradiating the foamable sheet with ionizing radiation to thereby crosslink the propylene resin, followed by heating the foamable sheet so as to cause the same to foam and simultaneously effecting molding thereof. This process produces a composite foamed molding product which is lightweight, having a level of strength barely enabling use as a base material (aggregate) and high rigidity, and which is suitable for use as an interior finish base material in automobiles and the like. The above publication describes that, for example, a trunk room material and a door trim can be obtained by the above process, however, the cover sheet suffers from fur collapse in the case of a fur-transplanted cover sheet (e.g., a fuzzy sheet and velvet), so that the feeling of the surface of the foamed molding product is not always satisfactory.
Therefore, the development is desired of a process capable of producing an inexpensive automotive molded roof material which not only is lightweight, having high rigidity, and excellent in dimensional stability, acoustic characteristic, molding environment, designability, skin surface feeling, mounting workability and recyclability but also ensures molding freedom.